There are essentially two types of feeders for use for after-treatment of sheets of paper in printing machines and copiers, viz. friction feeder and vacuum feeders. In friction feeder, individual sheets are picked from piles of sheets by the fact that a rotary feeding roll is abutted against and pulls the top sheet from the pile, wherein a subjacent friction block normally retains subjacent sheets of the pile. Friction feeders are robust and in general reliable in operation, but occasionally more than one sheet at a time may happen to be picked mistakenly. The feeding rolls may also leave marks in the sheets. In vacuum feeders, sheets are picked from piles by the fact that the top sheet of the pile is sucked against a conveyor belt for transportation of the sheet to subsequent further processing. The vacuum feeder has not the disadvantages mentioned above of the friction feeder, but the function thereof is more sensitive and a vacuum feeder is considerably more expensive than a friction feeder.
Another disadvantage of friction feeders is that when picking sheets, the top sheet is drawn against the closest subjacent sheet of the pile, the picked sheet being electrostatically charged, which often gives rise to problems in after-treatment steps. The sheets namely get a tendency to stick to each other as a consequence of the electrostatic charge, which hampers, e.g., putting together and adjustment of sheets into a sheaf having straight edges in a stapling and folding machine when stapling together the sheaf for the formation of a booklet. Since colour printing and colour copying have become more and more common, the described problem of electrostatic charging of the sheets has become even greater, since coloured paper has a greater tendency to be charged than paper with black-and-white print.
In large copiers and printing machines, it is previously known to separate sheets of paper in the paper magazines of the machines by blowing air from the side through the magazine. In doing so, air is supplied through central supply ducts to exhaust nozzles at the magazine or the magazines from a central fan unit situated at a distance from the magazine or the magazines.